The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In many well treatments it is necessary to inject a fluid into the well under pressure. If some or all of the fluid leaks out of the wellbore, this is termed “fluid loss”. If the treatment is one, such as drilling, in which the fluid is supposed to be returned to the surface, if some or all of the fluid does not return due to fluid loss, this is called “lost circulation.” Lost circulation is a decades-old problem, but there is still not a single solution that can cure all lost-circulation situations. There are many available products and techniques, such as polymer pills and cement plugs, to cure lost-circulation issues.
One of the simplest approaches is to add a lost circulation material (LCM) in the drilling fluid and/or in the cement or polymer system. LCM systems often contain fibers. One of the major advantages of using fibers is the ease with which they can be handled. There is a wide variety of fibers available to the oilfield. Most are made from natural celluloses, synthetic polymers, and ceramics, minerals or glass. All are available in various shapes, sizes, and flexibilities.
Fibers decrease the permeability of a loss zone by creating a porous web or mat that filters out solids in the fluid, forming a low-permeability filter cake that can plug or bridge the loss zones. Typically, a very precise particle-size distribution must be used with a given fiber to achieve a suitable filter cake. Despite the wide variety of available fibers, the success rate and the efficiency are not always satisfactory.
There is a need for fibers that are less sensitive to the sizes of particles in the fluids, that can block wide fractures even when the particles are small, that can survive changes in pressure, that can control spurt (the large amount of fluid often lost very quickly as the filter cake is being formed), and that can control the total fluid loss.
Those skilled in the art will appreciate that the use of fibers in the context of lost circulation during well construction is distinctly different from that associated with well-stimulation treatments such as acidizing and hydraulic fracturing. The principal differences between the two applications are associated with permeability. The goal of lost circulation control during drilling or primary cementing is to block the flow of wellbore fluids into the formation. This involves reducing the permeability between the wellbore and the formation. On the other hand, the goal of stimulation treatments is to increase the effective permeability between the wellbore and the formation. Thus, any stimulation treatments involving fibers should not result in a permeability decrease.
Another distinction between well construction and well stimulation is the fluid-flow direction. During drilling and primary cementing, fluid flow into the formation is generally to be avoided. The goal is to decrease the fluid-flow rate or stop it altogether. Conversely, stimulation operations are concerned with increasing the rate at which fluids flow out of the formation and into the wellbore.
A notable application of fibers in the context of hydraulic fracturing is proppant flowback control. Fibers are mixed with proppant in a way such that, when the well produces, the fibers prevent migration of proppant particles away from the fracture and into the wellbore. Yet the proppant pack containing fibers must remain permeable and allow efficient reservoir-fluid production. Such a condition would have no utility in the context of lost circulation control.